To meet the ever-increasing demand for electric power at lower consumer cost, utility plant designers are planning new facilities that will operate at steam pressures and temperatures well beyond those currently employed. To meet their target efficiencies of greater than 60% versus today's design efficiencies of around 47%, steam parameters must be raised to 375 bar/700.degree. C. from today's typical 290 bar/580.degree. C. conditions. Unfortunately, an economical superheater tubing alloy capable of meeting the advanced steam parameters does not currently exist. Examination of currently available boiler tube alloys reveals the difficulty in achieving the required combination of strength, corrosion resistance, thermal stability and manufacturability.
To meet the 38 MPa (developed by the 375 bar steam pressure) hoop stress requirement of the steam tubing, the 100,000 hour stress rupture life must exceed 100 MPa at 750.degree. C. (midradius tube wall temperature needed to maintain a 700.degree. C. steam temperature at the inner wall surface). Because coal ash corrosion is capable of degrading most superheater tubing alloys, utility plant designers have placed stringent corrosion allowances on candidate alloys. The total metal loss due to inner wall steam erosion and outer wall coal ash/flue gas corrosion must not exceed 2 mm in 200,000 hours (target life for the superheater section of the steam boiler operating between 700.degree. C. and 800.degree. C.). In addition, the tube size, primarily for purposes of economy, must not exceed 50 mm outer diameter (O.D.) and 8 mm wall thickness and optimally be less than 40 mm O.D. with a maximum wall thickness of 6 mm. Further, the alloy must be fabricable in high yield using conventional tube-making practices and equipment. This places a maximum constraint on work-hardening rate and yield strength of the candidate alloy range, which runs counter to the need for superior strength and stress rupture life at service temperatures.
To achieve the boiler tube strength requirement, ferritic and austenitic steels must be excluded and even nickel-base solid solution alloys lack adequate strength. One is required to consider gamma prime containing alloys even though virtually all gamma prime containing alloys lack adequate chromium to ensure satisfactory coal ash/flue gas corrosion resistance and few of these high strength alloys possess adequate formability to be made into tubing. Adding chromium can degrade the strengthening mechanism and if added in excess, depending on composition, can result in embrittling sigma, mu or alpha-chromium precipitation. To ensure acceptable resistance on the steam side to "downtime" stress corrosion cracking (SCC) and pitting beneath the scale during operation, a minimum nickel and, advantageously a minimum molybdenum content must be present in the alloy. Since 700.degree. C. to 800.degree. C. is a very active range for carbide formation and embrittling phase precipitation, alloying content in the nickel plus cobalt matrix must be precisely limited.
Field fabrication places further restraint on composition. Tubing must be amenable to bending and welding during superheater boiler construction, necessitating that the alloy be supplied in the annealed condition (lowest possible practical yield strength). Whereas the strength requirement during operation demands the highest possible strength at operating temperature, requiring that the alloy be given an aging heat treatment to achieve peak strength values. Dealing with these seemingly incongruous constraints and ultimately resolving this material challenge with a cost effective alloy is the object of this invention.